Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Qual é o Propósito do Indicativo de Chamada com uma Frequência atual de 4rph?
 ###Tabelas: CREATE TABLE competition_timeline (competition_timeline_id AUTO_INCREMENT PRIMARY KEY, Competition VARCHAR, 1970–71 VARCHAR, 1971–72 VARCHAR, 1972–73 VARCHAR, 1973–74 VARCHAR, 1974–75 VARCHAR, 1975–76 VARCHAR, 1976–77 VARCHAR, 1977–78 VARCHAR, 1978–79 VARCHAR, 1979–80 VARCHAR, 1980–81 VARCHAR, 1981–82 VARCHAR, 1982–83 VARCHAR, 1983–84 VARCHAR, 1984–85 VARCHAR, 1985–86 VARCHAR, 1986–87 VARCHAR, 1987–88 VARCHAR, 1988–89 VARCHAR, 1989–90 VARCHAR, 1990–91 VARCHAR, 1991–92 VARCHAR, 1992–93 VARCHAR, 1993–94 VARCHAR, 1994–95 VARCHAR, 1995–96 VARCHAR, 1996–97 VARCHAR, 1997–98 VARCHAR, 1998–99 VARCHAR, 1999–00 VARCHAR, 2000–01 VARCHAR, 2001–02 VARCHAR, 2002–03 VARCHAR, 2003–04 VARCHAR, 2004–05 VARCHAR, 2005–06 VARCHAR, 2006–07 VARCHAR, 2007–08 VARCHAR, 2008–09 VARCHAR, 2009–10 VARCHAR, 2010–11 VARCHAR, 2011–12 VARCHAR) CREATE TABLE defunct_callsigns (defunct_callsigns_id AUTO_INCREMENT PRIMARY KEY, Callsign VARCHAR, Area served VARCHAR, Band VARCHAR, Freq currently VARCHAR, Purpose VARCHAR) CREATE TABLE apertura_2004_standings (apertura_2004_standings_id AUTO_INCREMENT PRIMARY KEY, Place INT, Team VARCHAR, Played INT, Draw INT, Lost INT, Goals Scored INT, Goals Conceded INT, Points INT) CREATE TABLE 1983 (1983_id AUTO_INCREMENT PRIMARY KEY, Opposing Teams VARCHAR, Against INT, Date VARCHAR, Venue VARCHAR, Status VARCHAR) CREATE TABLE targets (targets_id AUTO_INCREMENT PRIMARY KEY, HD designation VARCHAR, Constellation VARCHAR, Distance ( ly ) VARCHAR, Spectral type VARCHAR, Signal power ( kW ) INT, Date sent VARCHAR, Arrival date VARCHAR)  
SELECT Purpose FROM defunct_callsigns WHERE Freq currently = 4rph